Polyolefin-based resins such as polypropylene and polyethylene are inexpensive and have various good properties such as good moldability, chemical resistance, water resistance, and electrical characteristics. Therefore, in recent years, such polyolefin-based resins are widely used for sheets, films, molded products, and the like. However, unlike polar base materials containing polyurethane-based resins, polyamide resins, acrylic-based resins, polyester resins, and the like, base materials containing the polyolefin-based resins are nonpolar and crystalline (hereinafter, these base materials containing the polyolefin-based resins may be referred to as “polyolefin base materials”). Therefore, these polyolefin base materials have a drawback in that application of polar paints thereto and their adhesion (bonding) to polar materials are difficult, i.e., a drawback in that their adhesion (bonding) properties are low.
A method to overcome this problem is that the surface of a polyolefin base material is activated by plasma or gas flame treatment to improve adhesion properties. However, this method has a problem in that its complicated process requires large equipment cost and involves time loss. A further drawback is that the effects of the surface treatment could be non-uniform when the shape of the base material is complexity or due to the influence of a pigment or an additive in the resin.
Another method is that a chlorinated polyolefin resin is used as a coating composition to improve the affinity of a polyolefin base material for a polar paint or a polar material so that the adhesion properties are improved. However, this method has a stability problem due to dehydrochlorination. In addition, because of recent increasing environmental awareness, there is a tendency to avoid the use of chlorine, and this is also a problem.
Paints and adhesives (active energy ray curable paints and adhesives) containing compounds (active energy ray curable compounds) that are curable with active energy rays typified by ultraviolet (UV) rays and electron beams (EB) have been developed. These active energy ray curable paints and adhesives have advantages, for example, in that they are solvent free and their environmental load is low, that their curing rate is very high and products can be produced at high productivity, and that they can be used also for thermally unstable materials because a heating process is not necessary. Various techniques for adhesion of active energy ray curable paints and adhesives to polar base materials are known.
Generally, resins capable of allowing paint or ink components to adhere to nonpolar base materials such as polyolefin base materials have low compatibility with active energy ray curable compounds. Therefore, generally, to apply an energy ray curable paint or ink to a polyolefin base material, the base material is undercoated (pre-coated) with a resin capable of allowing the paint or ink component to adhere to the base material, and then the energy ray curable paint or adhesive is applied.
In view of the above circumstances, a technology for improving the compatibility of resins capable of allowing paint and ink components to adhere to polyolefin base materials with active energy ray curable compounds is being developed. Known examples of such technology include: a composition containing a chlorinated polyolefin resin and a photo-curable compound (JP 2005-139305 A: Patent document 1); and an ultraviolet ray curable coating composition containing a predetermined amount of unsaturated carboxylic acid and/or a derivative thereof and also containing a modified polyolefin resin having a weight-average molecular weight within a predetermined range (JP 2003-238885 A: Patent document 2). However, in the technology in Patent document 1, occurrence of dehydrochlorination due to irradiation with ultraviolet rays and a reduction in temporal stability of adhesion caused by the dehydrochlorination are concerned. The technology in Patent document 2 embraces a problem that environmental load is high because substantially a large amount of a dilution solvent must be used.
Therefore, there has been a demand for a composition comprising an active energy ray curable compound and a chlorine-free resin, wherein the compound and the resin are uniformly dissolved or mixed in the composition without the presence of any solvent, and the composition exhibits good solution properties such as maintaining fluidity or a highly viscous state; and also the cured product of the composition provides a cured film firmly adhering to a polyolefin base material. However, such a composition has not been developed.